S. purpuratus ATPase and X. leavis cytochrome oxidase were purified to a state of homogeneity. Each was found to consist of eight polypeptides which were resolved by SDS-gel electrophoresis. Rabbit antiserum was then prepared against each enzyme complex. In order to determine the sites of synthesis of the peptides of each complex, isolated mitochondria were allowed to incorporate labeled amino acids in the presence of cycloheximide, solubilized with a non-ionic detergent, treated with the appropriate antiserum and the precipitates analysed by SDS-gel electrophoresis. The antigen-antibody precipitate obtained with ATPase antiserum contained two labeled peptides while that obtained with cytochrome oxidase antiserum contained one labeled peptide. Similar studies will be performed with other species of Xenopus and Sea urchin. It is hoped that it will be possible to identify species differences between corresponding mitochondrially made peptides. The mode of inheritance of these peptides in inter-species crosses will then be determined. This information should allow us to determine whether each peptide of each complex is encoded in nuclear or mitochondrial DNA.